The present invention relates generally to the connection of external cables to electrical apparatus and more particularly to a cable screen connection method and sleeved cable grommet for use in such a method.
Within electronic products, it is often necessary for equipment to directly connect to cables which then connect to remote equipment. In some applications, it is necessary for the cable to directly pass through the enclosure of the equipment without an intermediate connector and to directly connect to the circuitry within the enclosure. It is often necessary to provide screening for EMC (Electromagnetic Compatibility) performance around both the cable and the enclosure, an example being the use of braid or a metallic screen around the cable which then connects to a metallic surface on the inside of the enclosure to provide an overall screen for the equipment.
The conventional solution to the problem of connecting the cable screen to the enclosure screen has been to connect the cable screen onto a printed circuit board, or other electrical connection system, and then to connect onto the enclosure screen through some form of electrical contact. The effectiveness of the screening is reduced at high frequencies due to the electrical parasitic components introduced in the connection between the cable screen and the enclosure.
One prior technique is shown in U.S. Pat. No. 5,170,008 which describes a technique for connecting the braided shielding of a shielded cable to the frame of an EMI shielded metal enclosure by means of a grommet formed of an elastomer which includes a symmetrically formed metal band connecting the shield braid to the enclosure frame.
It is believed that an improved cable screen connection method would represent an advancement in the art.
It is, therefore, a primary object of the invention to provide an improvement in the cable connection art.
It is another object to provide an enhanced cable connection which can be effectively produced at reasonable costs and which can be readily practiced by those skilled in the art.
According to a first aspect of the invention, there is provided a method for providing EMC screening of a cable connection to an electrical apparatus comprising providing an electrically conductive screen layer on the internal wall of the enclosure adjacent the opening, providing a cable having a plurality of internal wires and a screen portion, folding back a portion of the screen portion along the cable, positioning the cable within a sleeved grommet comprised of an electrically conductive material having a conductive inner surface, the grommet being sized so as to compress the folded back screen portion of the cable against the conductive inner surface of the grommet, the grommet further including a groove within an outer surface thereof, and positioning the grommet and the cable within the opening of the enclosure so that the internal wall surrounding the opening fits within the groove to hold the grommet and cable in the opening and the conductive material of the grommet in direct electrical contact with the conductive screen layer on the internal wall of the enclosure.
According to a second aspect of the invention, there is provided a grommet for providing EMC screening of a cable connection to an electrical apparatus comprising an electrically conductive material of sufficient size so as to be compressed onto and provide electrical connection to a folded back screen portion of the cable, the grommet further including an external groove therein adapted for being positioned within the opening of the internal wall of the enclosure so that the grommet will be in direct electrical contact with the conductive screen coating on the internal wall.
The method and grommet of the present invention provides for a simpler and cheaper cable connection technique compared to the technique described in U.S. Pat. No. 5,170,008. The grommet of the present invention is so formed that there is no need to provide a metal band as is shown in the aforementioned patent. By avoiding the need for a metal band, this does away with a point of failure in the electrical connection. In addition, as will be apparent from the following description, the invention provides a convenient means of connection to the internal EMC screen of a plastic or other non-conductive equipment enclosure, the grommet of U.S. Pat. No. 5,170,008 designed for use instead with a metal (conductive) enclosure.
A wide range of suitable conductive elastomeric materials are available and the material chosen depends on the electrical environment in which the grommet is used. For computer applications operating with clock rates of several hundred megahertz, a silicone elastomer with a silver/glass conductive filler may be successfully used.
Thus in the present invention, the cable screen makes connection to a sleeved grommet component which is itself conductive and the grommet makes contact to the screen of the enclosure of the equipment to which the cable is connecting. In this way, a screen is maintained around the complete cable and equipment system which has the capability to be effective at a very high frequencies, e.g., greater than 100 MHz.